1. Field of the Invention
This invention relates to a process for the preparation of an encapsulated electrostatographic toner material, and more particularly relates to a process for the preparation of a pressure fixable encapsulated electrostatographic toner material.
2. Description of Prior Arts
There is known an electrostatography which comprises developing a tone electrostatic latent image contained on a photoconductive or dielectric surface with a toner material containing a colorant and fixing aid to produce a visible toner image, and transferring and fixing the visible toner image onto a surface of a support medium such as a sheet of paper.
The development of the latent image to produce a visible toner image is carried out by the use of either a developing agent consisting of a combination of a toner material with carrier particles, or a developing agent consisting of a toner material only. The developing process utilizing the combination of a toner material with carrier particles is named "two component developing process", while the developing process utilizing only a toner material is named "one component developing process".
The toner image formed on the latent image is then transferred onto a surface of a support medium and fixed thereto. The process for fixing the toner image to the support medium can be done through one of three fixing processes, that is, a heat fixing process (fusion process), a solvent fixing process and a pressure fixing process.
The pressure fixing process which involves fixing the toner material onto the surface of a support medium under application of pressure thereto is described, for instance, in U.S. Pat. No. 3,269,626. The pressure fixing process involving the use of neither a heating procedure nor a solvent produces no such troubles as inherently attached to either the heat fixing process or the solvent fixing process. Moreover, the pressure fixing process can be employed with a high speed automatic copying and duplicating process, and the access time is very short in the pressure fixing process. Accordingly, the pressure fixing process is thought to be an advantageous fixing process inherently having a variety of preferable features.
However, the pressure fixing process also has a variety of inadvantageous features. For instance, the pressure fixing process generally provides poorer fixability than the heat fixing process does, whereby the toner image fixed onto a paper is apt to rub off easily. Further, the pressure fixing process requires very high pressure for the fixing, and such a high pressure tends to break the cellulose fibers of the support medium such as paper and also produces glossy surface on the support medium. Moreover, the pressing roller requires to have relatively greater size, because the roller necessarily imparts very high pressure to the toner image on the support medium. Accordingly, reduction of the size of a copying and duplicating machine cannot exceed a certain limit defined by the size of a pressing roller.
There has been previously proposed an encapsulated toner material which comprises toner particles enclosed with microcapsules, so as to overcome the above-described disadvantageous features of the pressure fixing process. The encapsulated toner material is prepared by enclosing a core material (containing a colorant such as carbon black) with a shell which is rupturable by the application of pressure in the developing stage. Thus prepared encapsulated toner material has various advantageous features; for instance, fixing of the encapsulated toner material does not require very high pressure, but the fixability is high. Accordingly, the encapsulated toner material is viewed as suitable for the use in the pressure fixing process. However, the encapsulated toner materials proposed up to now appear unsatisfactory in practical use, because they fail to meet some of requirements required for providing smooth copying and duplicating operation and satisfactory toner image fixability and quality.
More in detail, it is required for the toner material for the use as a dry type developing agent in the electrostatography to have excellent powder characteristics (or, powder flowability) to provide high development quality, and to be free from staining the surface of a photosensitive material on which a latent image is formed.
Further, a toner material employed for the two component developing process is also required not to stain the surfaces of the carrier particles employed in combination. The toner material for the use as a developing agent in the pressure fixing process is furthermore required to be satisfactory in the fixability under pressure and not to undergo off-setting on the roller surface, that is, phenomenon that the toner adheres to the roller surface so as to stain it.
In summary, the toner material employed in the pressure fixing process ought to be at a high level in all characteristics such as powder characteristics (powder flowability), fixability onto a support medium (e.g., paper) as well as presevability of the fixed image, resistance to the off-setting, and electron chargeability and/or electroconductivity depending on the system employed. The previously proposed encapsulated toner materials are unsatisfactory in some of these characteristics.
For instance, the encapsulated electrostatographic toner material can be prepared in the form of a powder, as described above, by a process comprising a stage of forming resinous shells around micro-droplets of hydrophobic core material containing colorant dispersed in an aqueous medium to produce microcapsules therein, and a stage of separating the microcapsules from the aqueous medium through a drying procedure such as spray drying.
It has been proposed that in the process for the preparation of an encapsulated toner material, an emulsion stabilizer such as a hydrophilic polymer is introduced into the aqueous medium for stably dispersing the hydrophobic core material in the form of micro-droplets in the aqueous medium. However, the encapsulated toner particles obtained through spray-drying of the microcapsule dispersion produced in the presence of such emulsion stabilizer is liable to aggromerate to form secondary particles. Otherwise, although the encapsulated toner particles are present in a fine powdery form just after the spray-drying, these particles are liable to aggromerate to form secondary particles upon storage under high temperature-high humidity conditions or upon storage at room temperature in an atmospheric condition for a long period. Moreover, these toner particles are apt to aggromerate in a developing apparatus of the copying machine to form not a small amount of secondary particles. The formation of the secondary particles are highly disadvantageous because the secondary particles cause deterioration of resolution of the visible image developed by toner material.
Moreover, the encapsulated toner material prepared in the presence of the known emulsion stabilizer such as a hydrophilic polymer is liable to unfavorably vary its electric resistance and chargeability depending on temperature. The electrostatographic process employing such toner material is easily influenced by fluctuation of surrounding conditions such as temperature and humidity. Accordingly, it can hardly produce a visible toner image of stable quality.
It has also been proposed that a surface active agent is employed in place of the hydrophilic polymer for dispersing the hydrophobic core material in the aqueous medium in the form of micro-droplets. However, the use of a surface active agent is also liable to provide the encapsulated toner particles with increase of the temperature dependence of electric resistance and chargeability. Accordingly, the electrostatographic process employing such toner material is also easily influenced by fluctuation of surrounding conditions such as temperature and humidity. Accordingly, it still hardly produce a visible image of stable quality.